1. Field of the Invention
The present invention relates to a shadow-mask type color-generating cathode-ray tube (color CRT) and more particularly, to a shadow-mask type color CRT equipped with a slot type shadow mask, which reduces the moire phenomenon without deteriorating its vertical resolution.
2. Description of the Prior Art
As shown in FIG. 1, a shadow mask 18 of a conventional shadow-mask type color CRT has a number of slots 17 allowing selectively electron beams emitted from three electron guns to arrive at a phosphor screen through the mask 18. The three electron beams for red (R), green (G) and blue (B) colors are deflected by horizontal and vertical deflecting magnetic fields generated by a deflection yoke in the horizontal and vertical directions X and Y of a face panel.
The three electron beams are horizontally scanned over the entire phosphor screen. On the other hand, the beams passing through the slots 17 in the shadow mask 18 strike the corresponding phosphor stripes arranged in the entire phosphor screen and excite them. Thus, a wanted color image is displayed on the face panel.
The slots 17, all of which have the same roughly rectangular shape, are regularly arranged to form a mosaic in the horizontal and vertical directions X and Y of the face panel. In other words, the slots 17 are arranged in a staggered pattern. Between any vertically adjacent slots 17, a bridge area 19 is formed to maintain the mechanical strength of the shadow mask 18.
Specifically, a given number of the slots 17 are arranged at a fixed vertical pitch Pv in the vertical direction Y, forming a slot column 17a. Another given number of the slots 17 are arranged at the same fixed vertical pitch PV in the vertical direction Y, forming another slot column 17b. A plurality of the slot columns 17a and a plurality of the slot columns 17b are alternately arranged in parallel in the horizontal direction Y. The columns 17a and 17b are arranged at a fixed horizontal pitch PH.
The slot 17 in any one of the slot columns 17b is shifted in the vertical direction Y with respect to a corresponding one of the slots 17 in an adjacent one of the columns 17a by a half of the vertical pitch PV, i.e., (xc2xd)PV. In other words, the slots 17 are arranged so that, in any adjacent slot columns 17a and 17b, the horizontal center line of the bridge area 19 equally divides the opposed slot 17 into two in the vertical direction Y.
The fixed vertical pitch Pv is typically set at a value from 0.6 mm to 1.0 mm.
Each of the bridge areas 19 provides an electron-beam stopping area, which tends to create a shade on the phosphor screen, resulting in reduction in brightness. Also, the smaller the spot diameter of the electron beams, the larger the contrast on the phosphor screen, which makes the moire phenomenon remarkable.
To solve this problem, conventionally, the vertical pitch PV is varied to minimize the visibility of the moire within the shadow mask 18.
However, with the conventional shadow-mask type color CRT as stated above, when this CRT with the vertical pitch PV of 0.6 to 1.0 mm is used as a visual display terminal (VDT) in such an application as a computer display terminal, the shades produced by the bridge areas 19 tend to prevent the phosphor stripes from fluorescing at a part of the character and/or figure to be displayed, because the CRT requires a high resolution. This results in that the character cannot be read and/or the background and/or figure looks rough and non-uniform, making reading of the character still more difficult.
These shades further cause another problem of resolution deterioration.
Additionally, another conventional color CRT was disclosed in the Japanese Non-Examined Patent Publication No. 2-103847 published in April 1990. In this CRT, the vertical width BW of the bridge areas is varied at random under the condition of xcex94xe2x89xa7BW, where xcex94 is a shift for any adjacent slot columns.
Accordingly, an object of the present invention is to provide a color CRT equipped with a slot-type shadow mask that enables to reduce the moire phenomenon without deteriorating its vertical resolution.
The object together with others not specifically mentioned will become clear to those skilled in the art from the following description.
A shadow-mask type color CRT according to the present invention includes a shadow mask fixed apart from a face panel and opposed to a phosphor screen. The mask has slots allowing selectively electron beams to arrive at the phosphor screen through the mask. Each of the slots has an vertically elongated shape. The slots are arranged horizontally at a fixed horizontal pitch, and vertically at a fixed vertical pitch. At least two of the vertically adjacent ones of the slots have a bridge area therebetween. Each of the bridge areas serves as an electron-beam stopping area. In addition, at least of the two horizontally adjacent ones of the slots are vertically shifted at a fixed value equal to or greater than a half of the vertical pitch. The electron beams are scanned perpendicular to the slots. The vertical pitch of the slots is set at a value ranging from 0.2 mm to 0.5 mm.
With the color cathode-ray tube according to the present invention, the slots of the shadow mask, which have a vertically elongated shape, are arranged horizontally at a fixed horizontal pitch and vertically at a fixed vertical pitch. Also, horizontally adjacent slots are vertically shifted at a fixed value equal to or greater than a half of the vertical pitch. Further, the vertical pitch of the slots is set at a value ranging from 0.2 mm to 0.5 mm. As a result, the moire phenomenon can be reduced without deteriorating its vertical resolution.
In a preferred embodiment, each of the bridges has a fixed vertical length ranging from 0.02 mm to 0.08 mm. In this case, the moire phenomenon can be further reduced without deteriorating its vertical resolution.
In another preferred embodiment, the vertical pitch of the slots is set at a value ranging from 0.2 mm to 0.275 mm or from 0.2 mm to 0.28 mm corresponding to a raster (or screen) size of the tube. In this case, the moire phenomenon can be suitably reduced corresponding to the raster (or screen) size without deteriorating its vertical resolution.